1. Field of the Invention
The present invention pertains to laser detection and ranging (“LADAR”) systems, and, more particularly, to the pulse capture electronics of LADAR systems.
2. Description of the Related Art
A need of great importance in some military and civilian operations is the ability to quickly detect, locate, and/or identify objects, frequently referred to as “targets,” in a “field of view.” A common problem in military operations, for example, is to detect and identify targets, such as tanks, vehicles, guns, and similar items, which have been camouflaged or which are operating at night or in foggy weather. It is important in many instances to reliably distinguish between enemy and friendly forces. As the pace of battlefield operations increases, so does the need for quick and accurate identification of potential targets as friend or foe, and as a target or not.
Useful techniques for identifying targets have existed for many years. For instance, in World War II, the British developed and utilized radio detection and ranging (“RADAR”) systems for identifying the incoming planes of the German Luftwaffe. RADAR uses radio waves to locate objects at great distances even in bad weather or in total darkness. Sound navigation and ranging (“SONAR”) has found similar utility and application in environments where signals propagate through water, as opposed to the atmosphere. While RADAR and SONAR have proven quite effective in many applications, they are inherently limited by a number of factors. For instance, RADAR is limited because it uses radio frequency signals and large antennas used to transmit and receive such signals. Thus, alternative technologies have been developed and deployed.
One such alternative technology is laser detection and ranging (“LADAR”). Similar to RADAR systems, which transmit radio waves and receive radio waves reflected from objects, LADAR systems transmit laser beams and receive reflections from targets. In LADAR systems, brief laser pulses are generated and transmitted via an optical scanning mechanism. Some of the transmitted pulses strike a target and are reflected back to a receiver associated with the transmitter. The time between the transmission of a laser pulse and the receipt of the reflected laser pulse (a “return pulse”) is used to calculate the “range” from the target to the object that receives the return pulse.
Because LADAR provides range information, the data is “three-dimensional,” i.e., it provides information about the target in three dimensions. Typically, these dimensions are range, azimuth, and elevation. The shorter wavelengths of light signals (relative to radio signals) also provide much higher resolution and tighter beam control. These attributes of LADAR data greatly assist not only with target location, but also target identification. Thus, in many respects, LADAR systems can provide much greater performance than can, e.g., RADAR and SONAR systems.
The evolution of one particular LADAR system can be traced by reviewing the following issued U.S. Letters Patent:                U.S. Pat. No. 5,243,553, entitled “Gate Array Pulse Capture Device,” issued Sep. 7, 1993, to Loral Vought Systems Corporation as the assignee of the inventor Stuart W. Flockencier;        U.S. Pat. No. 5,357,331, entitled “System for Processing Reflected Energy Signals,” issued Oct. 18, 1994, to Loral Vought Systems Corporation as the assignee of the inventor Stuart W. Flockencier;        U.S. Pat. No. 5,511,015, entitled “Double-Accumulator Implementation of the Convolution Function,” issued Apr. 23, 1996, to Loral Vought Systems Corporation as the assignee of the inventor Stuart W. Flockencier; and        U.S. Pat. No. 6,115,113, entitled “Method for Increasing Single-Pulse Range Resolution,” issued Sep. 5, 2000, to Lockheed Martin Corporation as the assignee of the inventor Stuart W. Flockencier.These patents all describe the data acquisition electronics, or “pulse capture electronics” (“PCE”), of the LADAR system. They also disclose a LADAR transceiver whose operation is more fully disclosed in the following patents, among others:        U.S. Pat. No. 5,200,606, entitled “Laser Radar Scanning System,” issued Apr. 6, 1993, to LTV Missiles and Electronics Group as the assignee of the inventors Nicholas J. Krasutsky, et al.        U.S. Pat. No. 5,224,109, entitled “Laser Radar Transceiver,” issued Jun. 29, 1993, to LTV Missiles and Electronics Group as the assignee of the inventors Nicholas J. Krasutsky, et al.; and        U.S. Pat. No. 5,285,461, entitled “Improved Laser Radar Transceiver,” issued to Feb. 8, 1994, to Loral Vought Systems Corporation as assignee of the inventors Nicholas J. Krasutsky, et al.This LADAR system also processes the acquired data for a number of end uses, such as those more fully disclosed in a number of patents, including:        U.S. Pat. No. 5,644,386, entitled “Visual Recognition System for LADAR Sensors,” issued Jul. 1, 1997, to Loral Vought Systems Corp. as assignee of the inventors Gary Kim Jenkins, et al.        U.S. Pat. No. 5,852,492, entitled “Fused Lasar Range/Intensity Image Display for a Human Interpretation of Lasar Data,” issued Dec. 22, 1998, to Lockheed Martin Vought Systems Corp. as the assignee of the inventors Donald W. Nimblett, et al.; and        U.S. Pat. No. 5,893,085, entitled “Dynamic Fuzzy Logic Process for Identifying Objects in Three-Dimensional Data,” issued Apr. 6, 1999, to Lockheed Martin Corp. as the assignee of the inventors Ronald W. Philips, et al.Each of these patents is commonly assigned herewith to the assignee of this invention, Lockheed Martin Corporation.        
One concern with virtually all LADAR receivers is the “gain” of their detectors. The gain controls the amount of amplification applied by the detector to a return pulse when it is received. The gain should be commensurate with the intensity of the return pulse. If the intensity of the return pulse is high, then the gain of the detector should be low to avoid over-saturating the detector's components. On the other hand, if the intensity is low, the gain should be high to facilitate subsequent processing, although not so high that “noise” is reported as a return pulse.
Setting the detector's gain, however, is fraught with many difficulties. The intensity of return pulses can vary wildly from one moment to the next and creates difficulty in setting the gain for any particular intensity level. Setting the gain arbitrarily high unnecessarily risks over-saturating the detector's components for high intensity returns and erroneously reporting noise as returned pulses. On the other hand, setting the gain unnecessarily low risks missing low intensity returns.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.